This invention relates to the field of marking materials with lasers and more particularly to a method of vitrifying heat sensitive coatings to materials.
Bricks can be formed from a variety of clay-based materials and in a variety of colors and sizes. Primary materials include surface clays, fire clays, shales or combinations of these. For proper forming, such materials must have plasticity when sufficiently wetted, rigid when dried. Bricks can be then formed by extrusion, molding or dry-pressing and are fired in a kiln at temperatures between 1800xc2x0 F. and 2100xc2x0 F. (980xc2x0 C. and 1150xc2x0 C.) These variables in manufacturing produce units with a wide range of colors, textures, sizes and physical properties.
Naturally occurring clays are divided into specific types having particular properties. For example, clay is defined as a natural, mineral aggregate consisting essentially of hydrous aluminum silicate. It is a product of decomposition and alteration of feldspathic rocks and contains a mixture of particles of different sizes and widely differing physical, chemical and mineralogical properties. The non-plastic portion consists of altered and unaltered rock particles of which the most common and abundant substances are quartz, micas, feldspars, iron oxides, and calcium and magnesium carbonates. Organic matter usually is present in greater or lesser amounts, and frequently plays an important role in determining clay properties. The essential constituents of clays are hydrated silicates of aluminum, of which there are several, but the most important and widespread are the kaolinite group. The typical clay mineralsxe2x80x94kaolinite, montmorillonite, etc., have microscopic plate-like structures which are chiefly responsible for their plasticity (formability) when wetted with water. The fineness of a clay""s grain influences not only its plasticity but also such properties as drying performance, shrinkage, warping, strength and quality of marking achievable by laser energy. Clay""s with high aggregate contents, sands and organic matter are prone to poor glassy vitrification by laser energy. Clays occur in three principal forms, all of which have similar chemical compositions but different physical characteristics. Surface clays may be the upthrusts of older deposits or of more recent, sedimentary formation. As the name implies, they are found near the surface of the earth.
Shales are clays that have been subjected to high pressures until they have hardened almost to the form of slate. Fire clays are usually mined at deeper levels than other clays and have refractory qualities. As a rule, they contain fewer impurities than shales or surface clays and have more uniform chemical and physical properties. Surface clays and fire clays differ from shales more in physical structure than in chemical composition. Chemically, all three are compounds of silica and alumina with varying amounts of metallic oxides and other impurities. Although technically metallic oxides are impurities, they act as fluxes, promoting fusion at lower temperatures. Metallic oxides. (particularly those of iron, magnesium and calcium) influence the color of the finished fired product. The brick manufacturer minimizes variations in chemical composition and physical properties by mixing clays from different locations in the pit and from different sources. However, because clay products have a relatively low selling price, it is not economically feasible to refine clays to produce uniform raw materials. Since variations in properties of raw materials must be compensated for by varying manufacturing processes, properties of finished products from different manufacturers will also vary somewhat. The widespread usage of bricks as a building material on highly visible areas such as walkways and building fronts has led artisans to attempt to etch and decorate such materials with letters and/or graphical patterns.
It is known that glass can be formed by melting or fusing materials under extremely high temperatures by a process called vitrification. It is also known that lasers produce intensely focused beams of light at specific wavelengths which results in localized heating of an object which falls in the path of the active beam. Laser light can be produced and amplified by a variety of sources including, for example, Nd:YAG lasers produce laser light at a principal wavelength of 1064 nanometers (nm). Nd:YAG lasers can be operated as a continuous wave (CW) laser or with pulse or frequency modification. In the latter instance, a Q-switch is used to reflect the laser beam back into the lasing chamber to build up more power before the beam is released. The result is a pulsed laser with each pulse being more intense than a continuous wave beam from the same laser unit. Other lasers, such as carbon dioxide (CO2), can be constructed and configured for different wavelengths and power outputs.
Laser marking of bricks, pavers, terra-cotta tiles, and other high clay content materials is known in the art. U.S. Pat. No. 6,064,034 previously issued to the Applicant, the contents of which are incorporated herein by reference, discloses a process for the vitrification of bricks and other vitrescent objects by use of a laser. In particular, a continuous wave beam such as that provided from a ND:Yag or CO2 laser operating in a power range of 50-250 watts with a collimator and lens provides a laser beam intensity in the range of about 1.6xc3x97105-1.4xc3x97106 watts/cm2 for use in marking of a vitrescent object. The laser beam is steered by a computer to produce lettering and graphical patterns on the vitrescent object. For process efficiency and enhancement of the appearance, the surface of the vitrescent object is heated to a temperature of about 100xc2x0 F. prior to vitrification, or dried so as to achieve a predetermined moisture content.
U.S. Pat. No. 5,538,764 discloses a method of removing a surface layer from a hydraulically bonded material such as concrete. The disclosure is directed to the use of a laser having a power density between 100 W/cm2 and 800 W/cm2 causing surface layer removal between a depth of 1 mm to about 4 mm.
U.S. Pat. No. 5,554,335 is directed to the marking of ceramics by three distinct laser power conditions. A first laser power condition is carried out in a first finite time period, a second laser power condition is carried out in a second time period, and a third laser power condition is carried out in a third finite time period. This process requires an exceedingly long residence time with a concomitant loss of economic efficiency. Additionally, the quality of marking which results from the practice of this invention does not result in a smooth glassy appearance.
U.S. Pat. Nos. 4,769,310 and 5,030,551 discloses a method for marking ceramic materials, glazes, glass ceramics and other glasses. Ceramic and glass materials have been treated at high temperatures in their formation, and thereby have a glassy surface, or a glaze over their entire surface. Such glassified surfaces are difficult to mark, even with a laser. As a result, the ""310 and ""551 patents disclose a method of applying a transparent layer of material (e.g. 100 to 10,000 angstroms thick) such as titanium dioxide to the outer surface of the ceramic or glass object, and then irradiating the oxide layer with a pulsed laser beam. The irradiation causes discoloration of the applied oxide layer at the irradiated areas.
Glazed ceramic materials, such as whiteware, often develop regions of cracking due to wear, impacts, thermal stresses and the like. U.S. Pat. No. 5,427,825 discloses a method of repairing such glaze defects by preheating a glaze defect area with radiant energy. The glaze defect area is then treated by applying higher power radiant energy (such as infrared) from a laser to provide localized heating of the glaze material. The surface is further treated with radiant energy at a lower power density so as to limit the rate of cooling of the fusion zone and the immediately surrounding regions. This multi-step process prevents thermal stress cracking during the glaze defect repairing procedure.
U.S. Pat. No. 4,814,575 discloses yet a further method of surface-treating ceramic workpieces using a laser. A CO2 laser directs a beam onto a ceramic workpiece to be treated. A carrier gas injector injects a moderate throughput of spheroidized powder of a ceramic material into the beam. After melting and solidifying, the added layer has microcracks which are smaller than the cracks in the untreated surface. This method is useful in the construction of heat engines.
International Application WO 95/35269 discloses a method for the laser marking of bricks to produce artistic patterns, signs and symbols on the brick""s surface. This application fails to recognize those conditions which must be optimized in order to form a smooth, glassy marking on the brick.
U.S. Pat. No. 6,238,847 discloses a method of marking a surface of a substrate by applying a marking material having glass frit precursors to the surface of a substrate, irradiating a portion of the marking material with a laser beam to react the glass frit precursors with each other and to adhere to the irradiated marking material on the surface of the substrate forming a permanent marking thereon, then removing the non-irradiated portion of the marking material from the substrate. The marking material is applied in the form of a paste that may be formed from an inorganic pigment such as that sold by Cerdec which is a hydrous sodium lithium magnesium silicate. The pigment is mixed with water into a paste which is applied to the surface of a material. The problem is that the pigment stains porous substrates. The disclosure is limited to surface bonding of pigments applied to the surface with little or no surface damage. No preheating is necessary and a paper or plastic carrier can be used to hold the material, however, the power must be low to prevent the paper or plastic carrier from catching fire.
U.S. Pat. No. 6,075,223 discloses another marking process that requires the applying of a layer of glass frit material containing an energy absorbing enhancer to a glass substrate. The glass frit is irradiated with a laser beam having a wavelength selected to excite the energy absorbing enhancer forming a bonded and permanent marking layer atop the substrate. As with the previous patent, the glass frit is applied to the substrate surface at a desired thickness by use of hand-painting, mechanically brushing or rolling, pad or screen printing, or flood coating of the surface and then scraping or spinning to achieve the desired thickness. After irradiating, the excess material is removed. However, the pigment stains the substrate limiting use of the glass frit to certain substrates. The disclosure is for surface bonding at low power (1-30 watts) with a disclosed optimum power of 3-5 watts. The result is no surface damage as the material is applied atop of the surface. This eliminates preheating and is performed in a single pass. A paper or plastic carrier can be used to hold the material, however, the power must be low to prevent the carrier from catching fire.
U.S. Pat. No. 6,372,819 discloses a method of marking items with a laser activated coating composition. The composition has a dye and a binder resin which, when applied to a item that is heated with a laser, a polymeric material used in the composition is changed.
U.S. Pat. No. 5,801,356 discloses a method of marking a surface wherein a layer of material is adhesively applied to the surface of an item by heating at a temperature sufficient for inscribing the surface underlying the layer of material.
U.S. Pat. No. 5,673,532 discloses a method of coating a surface by applying tiles having edges to adhere to the surface and a vitrifiable grout is placed between the tiles and surface to allow vitrification of the grouting material to the surface.
U.S. Pat. No. 5,215,864 discloses a method of marking a metal material by use of dyes having an affinity to an oxide formed on the metal surface.
The prior art discloses methods of marking ceramic materials by vitrification and by use of various dyes bonded to the materials. However, the prior art fails to teach or otherwise disclose the use of dyes for use in vitrification where the colorant and products of vitrification are compounded together as a result of laser fusion. Nor does the prior art teach a carrier that eliminates the staining of the surrounding area, thereby eliminating problems with colorants that are inadvertently bonded to the surface of the material.
Accordingly, what is needed in the art is a selective laser compounding method that permits a colorant, such as a glass frit, metal oxide, or mixed metal oxide to be applied at high temperatures wherein the colorant and colorant carrier is vitrified to the material without staining of the surrounding area, or burning of the carrier.
The present invention provides a laser treatment method and process for coloring the surface of a substrate, effectively forming a new compound within the surface of a brick or similar vitrifiable substrate. The brick is formed from a clay-based material, which has at least a partial content that is vitrescent in nature. The present invention thereby uses a laser to heat specific areas of the surface of the brick to vitrify, or glassify, the material at specific locations including a colorant carrier. In one embodiment, the colorant carrier contains thereon a glass frit material, metal oxide, or mixed metal oxide with the carrier operating as an additional source of metal oxide. A laser beam is made steerable via computer controlled steering mirrors. Programs are then utilized to control the computer which steers the beam in the shape of letters or graphical characters across the face of the brick. The programs are written so that different methods of tracing letters or graphical patterns will optimize the laser beam width and intensity.
The preferred embodiment employs a Nd:Yag laser with a wavelength of 1064 Angstroms and utilizes a continuous wave (CW) beam rated for at least 50 Watts, a 10 inch or larger objective lens, and a beam telescope or collimator. A range of useful yet affordable powers would include 50-150 watt laser units. The collimator or beam expander, such as those available from Rodenstock Precision Optics, Inc., expands the beam in a range of 2xc3x97-8xc3x97 that of the original beam width emanating from the laser. The present inventor has discovered that modification of the standard collimator so as to produce a beam that is 1.6xc3x97 to 1.9xc3x97 will unexpectedly improve the resultant intensity thereby resulting in more efficient marking of objects, particularly in a mass production situation. In a most preferred embodiment, a beam expansion factor in the range of 1.6xc3x97-1.9xc3x97 is utilized with a 254 mm lens. The beam width must also be of an appropriate width so as not interfere with various rail components, and so that the beam properly interacts with the steering mirrors. As mentioned above, a Q-switch might be incorporated inside the Nd:Yag laser to cause a delay between laser pulses. This allows the power of the emitted beam to build up to a greater power density between each pulse. In its preferred embodiment, the present invention uses a CW beam because it has been found to produce a smoother marking pattern on the brick surface. It is contemplated that other lasers could also be used, including a CO2 laser which operates at a continuous wavelength of 10,640 Angstroms and a variety of power ranges.
A galvanometer motor is connected to a X-axis rotating mirror and yet another galvanometer motor is connected to a Y-axis rotating mirror. Each mirror is used in conjunction with the other, and with a computer control device, to steer and direct the laser beam in a pattern across the surface of a workpiece. Upon contact of the laser beam with the brick, a portion of the surface vitrifies, or turns to glass. The use of the colorant carrier exemplifies the instant process by allowing the use of bricks that otherwise fail to provide suitable coloration upon vitrification. The glass substance produced via vitrification can vary in color depending upon the color and type of brick used as well as the glass colorant. For instance, a reddish brick is found to produce a darker or black vitrification. Lighter shades of bricks, including for instance grey and ivory, have been shown to produce more of a greenish vitrification. In each case, the vitrified patterns are easily visible on the surface of the brick. The vitrification area and its appearance might also be varied by changing the laser type, laser configuration, and laser power, along with the brick type.
As the marking is vitrified and the glassification occurs at or below the substrate of the surface, the glassified surface area, inclusive of the colorant, is integrally formed into the surrounding material of the brick and cannot be readily worn off. While the lettering or graphical patterns are very visible, little to no channeling occurs in the brick surface to produce such patterns, and the pattern colors are a natural result of the glassification process. The vitrification process is more resistant to freezing or abrasion. For instance, paint or dye applied to the surface of a brick quickly wears off. Sand blasting requires generation of a mask and results in channels which must be painted in order to be readily seen. Such channels quickly fill up with such things as debris, dirt, rain, and/or snow, particularly when the bricks are laid flat and used on a walkway. Such channels can also be worn and xe2x80x9cislandsxe2x80x9d of material can be chipped away. Accordingly, the lettering or patterns become hard to read and the bricks must be frequently cleaned in order to maintain their original appearance and/or artistic purpose. The vitrification employed by the present invention, thereby creates a localized alteration, or glassification, of the material of the object to be treated. The color and clarity of the laser marking will vary depending on the softness and/or overall clay-content of the object to be treated. Clay is the base material which is altered by the heat of the laser to form glass. As a result, terra cotta, which is very soft and has a high clay-content, marks very well with a 50 Watt Nd:Yag laser. Other objects such as clay pots and pavers experience similar results as dependant upon similar factors. Alternatively, porous materials such as cement that have no clay content can be vitrified by use of clays in the mix wherein glassification occurs on the material although the clays and colorant carrier are required for the marking process.
It is therefore an objective of the present invention to provide an improved laser marking method for the localized vitrification of vitrescent objects.
It is a still further objective of the present invention to provide for the fast and efficient throughput of objects to be marked with the laser marking system even if the objects lack sufficient vitrescent properties.
It is yet another objective of the present invention to provide for programmable lettering or graphical symbols to be vitrified into the surface of vitrescent objects, such as bricks.
Another objective of the present invention to provide for programmable lettering or graphical symbols to be vitrified into the surface of non-vitrescent objects, such as cement.
It is a further object of the present invention to provide a laser marking method and process for the localized vitrification of vitrescent objects without the need for cleaning of colorants or glass frit from the object upon marking completion.
It is yet still another objective of the instant invention to provide a carrier that forms an integral portion of the composition for use in the vitrification process.
It is yet still another objective of the instant invention to provide nickle, brass, aluminum and the like metals capable of being placed into a foil that, upon absorption of laser energy, form reactive oxides or alterative compounds.
It is still another objective of the instant invention to teach the use of a sealed colorant package that allows prepackaging colorants to be used thereby eliminating the need for handling various pigments that can stain or are known carcinogens.
It is yet still another objective of the instant invention to provide enhanced laser marking of vitrescent objects, namely bricks, by utilizing optimal moisture content and surface temperature of the vitrescent object.
It is yet still another objective of the instant invention to provide enhanced laser marking of objects without staining of the surrounding area.
It is yet still another objective of the instant invention to provide enhanced laser marking of vitrescent objects at high temperatures without burning of the carrier.
Still another objective of the instant invention is to provide a prepackaged composition for vitrified marking of objects wherein unused composition is sealed with the package.
Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.